This invention relates to a method for anaerobic digestion of organic sludge such as sewage sludge and sludge from agricultural and fish waste products
Conventionally, raw sludge, excess sludge and the like which are formed in sewage treatment plants have been treated by dehydration followed by incineration after which they have been discarded. However, in recent years, it is becoming more common to treat sewage by the anaerobic digestion method. The anaerobic digestion method is one in which so-called methanogens are propagated in an anaerobic state, whereby the organic substances in the sewage are broken down primarily into methane gas. As a result, the amount of solids in the sludge is decreased. At the same time, methane gas, which can be utilized as a fuel, is obtained. The anaerobic digestion method has many other advantages such as that it consumes little energy, the mortality of pathogenic bacteria is high, and the digested sludge can be easily disposed of. However, the rate of propagation of methanogens is extremely low, so this method has the drawback that treatment of sludge requires much time.
There have been many different proposals for increasing the speed of anaerobic digestion. For example, an article in Kankyo Gijutsu (Environmental Technology) Magazine (Vol. 13, No. 11, pp. 772-780 (1984)) discloses a method in which excess sludge which is formed in an activated sludge tank is first treated by acid treatment, alkali treatment, or heat treatment, after which it undergoes anaerobic digestion. Japanese Patent Application Laid Open No. 58-72600 discloses a method in which sludge is first solubilized by ultrasonic treatment, after which anaerobic digestion is carried out.
However, even when excess is subjected to such pretreatments, it takes a long retention period of 15-20 days to achieve a digestion rate of around 50%. Furthermore, even if the retention period is further increased, the upper limit on the digestion rate is only about 55%.
The same problems are encountered when it is attempted to perform anaerobic digestion of waste products from factories for processing agricultural and fish products such as starch factories and food product factories, livestock feces and urine, livestock carcasses, fish entrails and bones, and the like. For this reason, there is still very little use of anaerobic digestion in these fields.
Thus, as described above, conventional anaerobic digestion methods suffer from the problems of being inefficient, of therefore requiring a large digestion tank, and of being unable to produce an sufficient decrease in the amount of solids.